Autism spectrum disorder is a major worldwide public health problem affecting 1 in 88 children and adults in the U.S. Autism is a lifelong debilitating developmental brain disorder characterized by impairments in social interaction and communication. We previously studied adults with autism using MRI diffusion tensor tracking (DTT), and found unique changes in white matter pathways used for recognizing faces and face emotions (critical aspects of social interaction and non-verbal communication). There was a relationship between these brain changes and early-childhood behaviors, suggesting that these brain changes may occur early in childhood, and may represent a lack of hemispheric specialization of face-processing pathways. In the same adults, we also found brain changes in the corpus callosum, a brain structure that allows information to flow between the brain's right and left hemisphere. Some characteristics of face processing and language function may be caused by such changes in the corpus callosum. However, to better understand the neurobiology of autism, it is necessary to test for these brain changes in very young children diagnosed with autism, because the changes in the adult brain could be due to many other factors that accumulate over decades. To determine whether such changes are a fundamental, primary abnormalities in autism that occur early in development, and that are present at the time of diagnosis, we will use DTT to: 1) test for water diffusion changes in the same face processing pathways in toddlers and preschoolers recently diagnosed with autism, 2) test for changes in language pathways, and 3) test for changes in the corpus callosum. The results will be compared to measures of face perception and language processing to determine the functional significance of the brain changes. We plan to study 20 toddlers/preschoolers with autism (2-5 years of age) and 20 individually-matched comparison children, where both groups have been clearly diagnosed and tested as either having autism or having normal development. The results will determine if the brain changes seen in adults with autism are more severe, similar, or less severe in early childhood soon after diagnosis. Our long- term goals are to: 1) provide a better understanding of the underlying biological mechanisms and causes of autism; 2) improve the early diagnosis, screening, risk assessment, and design/planning of early treatments and interventions; and 3) provide safe, non-invasive measures (biomarkers) that can be used to aid diagnosis and guide the search for genetic/immunological/environmental factors in autism.